Conventionally, a magnetic frequency triplomultiplier, whose typical structure is shown in FIG. 5 for use with a three-phase input and a single-phase output, has been applied to power sources for furnaces to melt metal and the like. The capacity of such frequency triplo-multiplier is increasing these days, and a unit capacity of up to 6,000 KVA has been built recently. The illustrated frequency triplo-multiplier uses three saturation reactors ST-1, ST-2, ST-3 for three phases of a three-phase input. Their primary windings W.sub.in -1, W.sub.in -2 and W.sub.in -3 are connected in .DELTA., while their secondary windings W.sub.o -1, W.sub.o -2 and W.sub.o -3 are connected in series. Serial inductances L.sub.in -1, L.sub.in -2 and L.sub.in -3 and parallel capacitances C.sub.in -1, C.sub.in -2 and C.sub.in -3 are connected to the input side of the primary windings so as to bring about resonance with the fundamental frequency of the input. A parallel capacitance C.sub.o is connected to the output side of the frequency triplo-multiplier so as to bring about resonance with the third harmonics, i.e., the triple frequency.
The operating principles of the frequency triplo-multiplier of the above construction is as follows; namely, the three cores ST-1, ST-2 and ST-3 are saturated by three-phase input voltages V.sub.in -1, V.sub.in -2 and V.sub.in -3, and harmonics voltages are generated therein. Triple frequency component voltages V.sub.0 of the harmonics, which are zero-sequence components, are extracted from the secondary side of the frequency triplo-multiplier. The illustrated embodiment has shortcomings in that a large number of harmonics are present in the input current, so that the input current is considerably distorted thereby causing adverse effects on the source of the three-phase input, and that loss is high and efficiency not very good. The above shortcomings are intensified with the increase of the capacity of the frequency triplo-multiplier, resulting in many practical problems.
To solve the above-mentioned shortcomings of the conventional structure, the inventors have proposed a modification of frequency triplo-multiplier as shown in FIG. 6. In this modification, an input resonant circuit similar to the preceding example is placed between the three-phase input voltages and the .DELTA.-connected primary windings of linear reactors LR and saturation reactors SR, which reactors are in series in each phase. The secondary windings of the serially-connected linear reactors LR and saturation reactors SR are also connected in series, so that a single-phase output voltage V.sub.o is extracted through an output resonant circuit C.sub.o L.sub.o. This modification has succeeded in reducing the distortion factor of the input current waveform to a level of one-fifth of the prior art device, but the number of reactors used is doubled to cause a 50% increase of the weight and a corresponding increase in cost.
As a means for solving the problems of weight and cost in the above modification in the past, the inventors have proposed a second modification of the frequency triplo-multiplier as shown in FIG. 7. In the second modification, an input resonant circuit similar to the preceding example is placed between three-phase input voltages and .DELTA.-connected windings of three-legged iron cores T. Each iron core T has a central leg N.sub.L with a gap and two side legs N.sub.s, N.sub.z, and windings on the central leg N.sub.L and one side leg N.sub.s are connected in series in each phase to form the above .DELTA.-connected windings. The windings on the remaining side legs N.sub.z of three-legged iron cores for the three phases are connected in series, so as to extract a single-phase output voltage V.sub.o by way of an output resonant circuit C.sub.o L.sub.o. Thus, the number of reactors in the second modification is the same as that of the conventional structure of FIG. 5. Thus, the increase of weight due to the use of many reactors in the first modification has been eliminated in the second modification, while the above-mentioned reduction of the distortion factor of the input current waveform to a level of one-fifth by the first modification is retained in the second modification. However, the second modification has a shortcoming in that its structure is somewhat complicated due to the use of a number of iron cores with comparatively complicated structure, and a room is left for further improvement.